Convexity and Parasagittal Versus Skull Base Meningiomas

Meningiomas

Meningiomas are the most common benign intracranial tumors, accounting for up to 13% to 26%. They originate from meningothelial cells present in the arachnoid layer of the meninges and are found over the convexities or in the skull base. Although most are benign, a few are classified as atypical (6%) or anaplastic (4%). They tend to occur anywhere arachnoid cells are found in the central nervous system, but they can occasionally be intraosseous. These tumors can be solitary, multiple, or can occur as an extensive thickening involving the meninges (meningioma en plaque).

With the advent of technology and ever-improving neuroimaging devices, meningiomas are diagnosed more than ever before. Historically, this was done using x-rays and angiograms, but CT and MRI scans have improved diagnosis significantly. This drastic change is reflected in the huge number of incidental meningiomas seen by the modern-day neurosurgeon.

Meningiomas are typically slow growing and are usually associated with a protracted clinical course. Virchow was the first to describe the classic pathologic feature of the meningioma tumor, with the term psammoma body (sandlike, rightly describing the granules noted within the tumor). Harvey Cushing first used the term meningioma in a 1922 publication describing tumors that originate from the meningeal (dural) coverings of the brain and spinal cord.

Donald Simpson's 1957 paper described the correlation between surgical resection of meningiomas and the rates of symptomatic recurrence:

• Grade I: Macroscopically complete resection of tumor, with excision of dural attachment and removal of abnormal bone (involved venous sinus is also excised in relevant cases)

• Grade II: Macroscopically complete resection of tumor with coagulation of dural attachment

• Grade III: Resection of tumor without coagulation or excision of dural attachment

• Grade IV: Partial debulking of tumor

• Grade V: Biopsy or decompression only

In his report, the risks of eventual recurrence after Simpson grades I, II, III, IV and V were 9%, 16%, 29%, 39%, and 99%, when the patients survived for 6 months or longer after surgery. It is important to note that Simpson's grading system was formulated in the pre-CT era and more highly specialised diagnostic aids have been developed since then. Several studies have advocated the use of MIB-1 for anticipating tumor recurrence. Oya and colleagues indicated that an MIB-1 LI value of 3% or higher was associated with a greater recurrence rate of World Health Organization (WHO) grade I meningiomas surgically treated by Simpson grade II or III resection. There is still no way of quantifying the MIB-1 value intraoperatively, as an integration of this with surgical resection might be beneficial in the surgical treatment of meningiomas.

Epidemiology

Meningiomas constitute approximately 18% to 20% of all intracranial tumors, with an annual estimated incidence of 2.3 cases per 100,000 persons. Intracranial meningiomas represent 98% of all CNS meningiomas, and spinal types make up the remaining 2%.

Autopsy studies showed meningiomas occurred in as much as 3% of the population over 60 years of age. There is a slight female preponderance (the female-to-male ratio is 1.8 : 1), and the incidence peaks during the fourth decade of life.

Meningiomas are less often seen in the younger age groups. Less than 2% occur in childhood and adolescence, whereas about 20% of cases seen in adolescents are associated with neurofibromatosis type I.

Based on their location, parasagittal meningiomas are the most common type, whereas convexity meningiomas are the second most common (15%). Approximately 5% to 15% of patients with meningiomas have multiple meningiomas, especially those diagnosed with neurofibromatosis (NF) 2. The age at diagnosis has been reported to be a significant predictor of survival for patients with benign, atypical, and malignant meningiomas.

Treatment Options

Surgical resection remains the gold standard for meningiomas; however, maximal resection the first time is the best. In some cases, a residuum has to be left due to proximity of vital structures/eloquent regions or tumor extension into inaccessible areas. Radiotherapy treatment has proved to be effective in controlling tumor growth, particularly for residual tumor following resection of skull base meningiomas. It is usually indicated for inaccessible meningiomas, inoperable meningiomas, postresection cavity for grade III meningiomas, or for remnants that have demonstrated confirmed growth. Although there is a lot in the literature regarding the management of meningiomas, there is sparse evidence for the radiation treatment of WHO grade II or III meningiomas.

The endoscopic approach is particularly useful for anterior fossa floor meningiomas. However, the more lateral a meningioma in this region extends, the more challenging the procedure is via this approach.

Embolization

Manelfe and associates, in 1973, first described the microcatheter technique of meningioma embolization. Preoperative embolization can be an important adjuvant treatment modality. From the literature, however, there appears to be no specific indications. Surgeons' preference, institutional practices, large tumor size, extensive tumor vascularity, skull base meningiomas with difficult-to-access arterial supply all played a role in patient selection. Certain benefits have been highlighted (ie, reduced operative blood loss, easier tumor resection, and shortened surgical time). Embolizing materials could be liquid or particulate agents.

The liquid agents include N-butyl cyanoacrylate (NBCA), onyx (ethylene vinyl alcohol polymer dissolved in dimethyl sulfoxide), and fibrin glue, whereas the particulate agents used are PVA particles and microspheres. Convexity meningiomas have their blood supply usually derived from external carotid artery feeders, and are more easily embolised.

Surgical Resection

The subfrontal approach is usually more suitable for large and giant olfactory groove meningiomas, as it allows for better exposure of the tumor and the ethmoidal area and it facilitates a good closure of the cranial base. The supraorbital craniotomy through an eyebrow incision can be used for small unilateral olfactory groove meningiomas. This is more favourable if the frontal sinus is not extensive. The advantage of using the pterional approach includes early visualization of the supraclinoid artery, anterior cerebral arteries, optic nerves, chiasm, and so on, making the posterior dissection of the tumor easier. The pterional approach is particularly useful for sphenoid wing or temporal meningiomas. This approach does not allow good visualization of the contralateral side, so it is usually reserved for medium-sized tumors. For other meningiomas, approaches are tailored to the location and can also be customized according to the surgeon's preference. Skull base meningiomas, particularly in sites like the petroclival/petrous ridge, tentorium, sigmoid, internal auditory canal and jugular foramen regions can be accessed using the middle fossa, retrosigmoid, translabyrinthine, transcochlear, infratemporal fossa and retrolabyrinthine approaches. Radiological imaging should be carefully studied preoperatively to decide on the shortest and safest route. Exquisite skill in navigating a narrow surgical corridor and excellent knowledge of the skull base anatomy (and the blood vessels and nerves that will be encountered) is compulsory.

Endoscopic sellar surgery was pioneered by Jankowski in 1992. Since the 1990s, the endonasal transsphenoidal approach has been increasingly used for planum sphenoidale meningiomas and other meningiomas in the sellar/parasellar region. Proponents of this approach highlight its advantages as including minimal brain retraction and better visualization of the suprasellar, parasellar, retrosellar, and retroclival regions.

Fractionated External Beam Radiation Therapy (EBRT)

Studies have shown that for atypical meningiomas where radical resection could not be achieved, external beam radiation therapy (EBRT) with a high dose of about 60 Gy could be beneficial. Some authors have even advocated the use of EBRT regardless of the extent of resection.

For malignant meningiomas, Milosevic and colleagues and Dzuik and colleagues provided evidence of surgical benefit followed by EBRT. More importantly, they demonstrated the need for EBRT to be administered initially rather than at progression. The use and extent of a margin in radiation therapy are topics of marked interest when comparing EBRT and SRS for malignant meningiomas.

Stereotactic Radiosurgery

Stereotactic radiosurgery was first utilised by Lars Leksell in the 1960s, but has become more increasingly used since the 1980s. It is considered most effective for tumors less than 3 cm in diameter or 10 cm ³ in volume. Other factors that influence whether or not stereotactic radiosurgery is used include clear tumor-brain interphase, proximity to areas of functionally important brain or nerves, and other critical structures.

Excellent tumor control for WHO grade I meningiomas is usually achieved with 12 to 16 Gy. Ganz and associates noted that a minimum peripheral tumor dose of 10 Gy was associated with higher failure risk, compared with a dose of at least 12 Gy. Stafford and colleagues reported no reduction in local control at 5 years with tumor margin doses of less than 16 Gy as compared with doses greater than or equal to 16 Gy. Along the same lines, Kondziolka and colleagues reported no improvement with marginal doses greater than 15 Gy versus less than 15 Gy.